This invention relates to an electron tube type unidirectional optical amplifier for amplifying light propagating in one direction, said electron tube type optical amplifier being applicable to various fields such as electric engineering, electronic engineering, quantum electronics, opto-electronics and laser engineering.
There have been proposed various lasers and travelling wave tubes for performing a unidirectional amplification of light.
Recently, gas laser, solid state laser, liquid laser and semiconductor laser have been practically used. These lasers are typical opto-electronic elements or devices which can perform the light generation and light amplification. In these elements or devices, energy of electrons bound by atoms and molecules in laser materials is used, and thus both forward wave and backward wave are amplified, and the optical amplification can not be performed in a unidirectional manner. Therefore, when light emitted by a laser is reflected by surfaces of lenses, optical fibers and optical disks and is made incident upon the laser, the thus returned light, i.e., back light, is also amplified. This makes laser emission quality and laser amplification quality unstable and generates excess noise.
Up to now, in order to solve the above problem, it has been generally proposed to provide an optical isolator between a laser light source and an optical system such that light reflected by the optical system is not made incident upon the laser. But, since the optical isolator has a bulk mainly made of a magnetic material and is very expensive, the application of the optical isolator is limited. In practice, the optical isolator has been used in a basic study of optical fields and large capacity optical fiber communication systems. However, the optical isolator could not be used in the field of optical disk devices which are small in size and less expensive in cost. Therefore, in the optical disk devices, the degradation of laser quality and the generation of noise due to the back light have been a technical obstacle to the application of lasers.
There has been further proposed an optical integrated circuit, in which a laser generating part, a light amplification part and a light modulating part are integrated as a single integrated unit, and information is processed at a high speed by light. However, such an optical integrated circuit has a problem that the various parts can not be effectively coupled with each other due to the back light from a succeeding part.
A free electron laser has been developed as a device for generating light within a wide wavelength range. The free electron laser operates on a principle which is entirely different from other lasers. In the free electron laser, energy of an electron beam travelling in one direction within a vacuum is given to light, and thus only a light component travelling in the same direction as the electron beam can be amplified. However, since the free electron laser has been developed mainly for generating light, it is not designed to utilize the above mentioned unidirectional amplification characteristic. Moreover, in the free electron laser, an exciting voltage for the electron beam is very high, such as not less than 10 MV, and an extremely high magnetic field is required to vibrate the electron beam. In this manner, the free electron laser has been developed for special high energy applications, and it would not be preferably applied to the electronic field of signal amplification.
A travelling wave tube is a unidirectional electron tube which has an operation frequency higher than the upper limit (about 1 GHz) of the operation frequency of normal electron tubes and transistors operating as a functional electron element having unidirectionality. In this travelling wave tube, a travelling velocity of an electromagnetic wave is decreased by means of a transmission delay line made of a metal, and energy of an electron beam emitted from an electronic gun is given to this electromagnetic wave. Energy loss due to scattering of electrons by collision with to surrounding materials is suppressed by evacuating a space surrounding the electrons.
In this travelling wave tube, the electromagnetic wave is amplified when the velocity of the electron beam coincides with the travelling velocity of the electromagnetic wave, and therefore the electromagnetic wave travelling in an opposite direction is not amplified. Since a wavelength of the electromagnetic wave is decreased in accordance with an increase in its frequency, an upper limit of the frequency of the travelling wave is imposed by a metal processing technique. Therefore, a frequency higher than several tens of GHz (wavelength less than several cm) can not be realized. Consequently, it is impossible at present to manufacture a travelling wave tube which can be applied to light having a wavelength not larger than 1 .mu.m due to the practical limit of the presently developed metal processing engineering.
To solve the above problems, the inventor of the present application has suggested a unidirectional optical amplifier using an electron beam in a solid state body in a co-pending Japanese Patent Application No. 9-71147 (corresponding to co-pending U.S. patent application Ser. No. 09/046,508 and European Patent Application No. 98 302 257.5). The inventor has theoretically proved that the unidirectional optical amplification is possible by combining an electron beam travelling line for an electron beam emitted into the solid state body with a delay waveguide made of a dielectric material for delaying light to be amplified.
In the above mentioned unidirectional optical amplifier, when the electron beam travelling line is made of ZnSe, a sufficiently high accelerating voltage can not be used, because when the accelerating voltage exceeds 2.5V, electrons can not travel along the travelling line. Then, a spatial phase variation of the electromagnetic field becomes very fine, and the delay waveguide has to be formed precisely, with a precision less than on the order of a nano-meter. At present such a precise processing can not be easily realized.